It is known that solid carbonaceous fuels are one of the cheapest and most abundant sources of energy. Currently, coal production provides a majority of the electricity produced in the United States, about 52%, while more than 70% of the electricity produced in India and China is generated from coal. Because of the substantial resources of coal in the United States, it is projected that the United States coal reserves will last about 250 years at the current consumption rate, and coal's share in the world's energy usage will continue to be significant over the next several decades. As the energy demands of the world grow, the coal consumption rate is expected to increase.
Conventional coal-based power generation technologies suffer from Carnot constraints that ultimately result in low conversion efficiencies, where they ordinarily require multiple processing steps to convert the chemical energy of coal to electricity. Since air is employed for the combustion of coal in these processes, the flue gases typically contain 10-15% CO2, with the remaining being nitrogen, where the nitrogen and CO2 then need to be separated by expensive and energy intensive processes in order to capture the CO2. Typically, sub-critical coal fired power plants operate with efficiencies of 33-35%. More advanced coal technologies have slightly improved efficiencies that may reach up 38% for ultra-super critical and 42% for integrated gas combined cycle (IGCC) processes. IGCC processes employ pure oxygen, instead of air, for gasification. Since no nitrogen enters the process stream, the flue gas is primarily made of CO2, and the separation step is not needed to capture the CO2. However, there are known to be expensive technologies with capital costs in excess of $1700/kW without CO2 capture and more than $2200/kW with CO2 capture.
IGCC technology addresses and improves both the conversion efficiency and ease of CO2 capture, at the expense of separating oxygen from air prior to gasification. Although it offers only a modest gain in efficiency (to around 40-42%), IGCC consumes large amounts of water required for the steam gasification step to produce a mixture of CO and hydrogen. Typically, 60-70% of the product stream from this process is made of CO2 and H2O, with the remainder being CO and hydrogen.
FutureGen (a public-private partnership to design, build, and operate the world's first coal-fueled, near-zero emissions power plant) and other integrated gasification fuel cell (IGFC) systems currently under development take advantage of the IGCC approach by combining the process line with a solid oxide fuel cell (SOFC) to improve the overall conversion efficiency of the system. One serious consequence of theses technologies is that these approaches require the consumption of large quantities of water for the coal gasification step. Water is a precious natural resource that is not readily available in sufficient quantities at every geographic location. Further, unprocessed water also lacks the necessary quality required for the gasification step in IGCC, FutureGen and IGFC processes. Water used in the gasification step needs to undergo expensive pretreatment for purification, which adds another undesirable cost factor. Moreover, all three of these processes require the use of oxygen during the gasification step to provide the heat necessary to drive the highly endothermic gasification reaction.
Efforts to address global climate change are in place requiring that the CO2 that is normally generated during the combustion and/or gasification processes in coal-based power plants must be removed from the exhaust gases and stored indefinitely in order to slow down the rate of increase in the level of carbon dioxide in the atmosphere. Carbon capture and sequestration (CCS) is likely to be required of all new coal-based power plants. Regulations could require that all existing power plants be retrofit with a CCS system.
Carbon capture involves separating the CO2 from the exhaust gas of a power plant before releasing the gas to the atmosphere. Geologic formations can provide permanent storage sites for CO2. Primary targets for geologic sequestration of CO2 include petroleum reservoirs, saline aquifers and deep, unminable coal seams. A recent DOE survey has identified sites with the potential to store over 3,500 billion tons of CO2.
Considering that all coal-based power plants emit copious amounts of the green house gas CO2 into the environment at the rate of about 2.7 tons of CO2 for every ton of coal consumed, there is a need to develop technologies that increase the efficiency of coal conversion, thereby reducing CO2 emissions with a proportionate amount. Further, considering the ever-decreasing availability of water and the need for expensive purification, there is a need to develop technologies that do not require water for these expensive processes.